Friday, December 9, 2011

LROC: Ejecta blanket geology

Unconsolidated material and impact melt (smoother areas) in the
continuous ejecta blanket of Stevinus A, northeast of the bright
nearside crater. LROC Narrow Angle Camera (NAC) observation M170227051L,
orbit 10220, September 9, 2011, with a native resolution of 0.53 meters
per pixel of a field of view 530 meters wide. View the full size
original LROC Featured Image HERE [NASA/GSFC/Arizona State University].

Stevinus A (31.75°S, 51.55°E) is an 8 km diameter Copernican-aged crater located south of Mare Fecunditatis. Stevinus A is exciting to lunar geologists specializing in the study of impact craters because of its young age, but being young is not the only special thing about this impact crater. Dry debris flows create braided stream-like deposits down the crater walls. A small impact melt pond and subsequent slumped material are observed on the crater floor. Furthermore, taking a look outside the crater rim, the continuous ejecta blanket surrounding the crater is composed of intermingled unconsolidated ejecta (blocky material) and impact melt streamers.

LROC Wide Angle Camera mosaic of two observations covering the environs
of Stevinus A. Of the Copernican Age, Stevinus A is geologically
fascinating because of the degree of crater freshness, the presence of
impact melt and very young dry debris flows inside the crater. LROC WAC
observations M128942557C and M128949353C, orbits 4135 and 4136, May 19
2010; resolution 55.6 meters per pixel from 39.2 kilometers
[NASA/GSFC/Arizona State University].

The opening image highlights the very quick nature of impact and subsequent ejecta emplacement. Sometimes, it is difficult to tell when ejecta and impact melt emplacement occurred. For example, was the unconsolidated material deposited first, followed by impact melt? Were melt and unconsolidated ejecta deposited simultaneously? In today's Featured Image, it is difficult to tell, especially because the impact melt streamers are not substantial flows as observed elsewhere on the Moon. However, based on observations of other Copernican-aged impact craters with impact melt, it is probable that the melt occurring in the Stevinus A ejecta blanket was emplaced after the ejecta blanket was emplaced - even if by only a few milliseconds. However, to be certain, scientists would need to complete a rigorous survey of these craters and make careful observations, attempting to discern the stratigraphic relationships between the unconsolidated ejecta and the impact melt.

Can you find additional evidence that supports (or refutes!) the observation that the impact melt was emplaced after the blocky portion of the ejecta blanket in the full LROC NAC image?